A number of patents have described the use of acoustic energy in droplet ejection. For example, U.S. Pat. No. 4,308,547 to Lovelady et al. describes a liquid drop emitter that utilizes acoustic principles in ejecting liquid from a body of liquid onto a moving document for forming characters or bar codes thereon. Lovelady et al. is directed to a nozzleless inkjet printing apparatus wherein controlled drops of ink are propelled by an acoustical force produced by a curved transducer at or below the surface of the ink.
The Lovelady et al. patent makes use of a piezoelectric shell transducer to both generate and focus the acoustic energy. Several other methods have also been developed to focus the generated acoustic energy and eject a droplet of liquid. For example, acoustically illuminated spherical acoustic focusing lenses as described in U.S. Pat. No. 4,751,529 to Elrod et al. and planar piezoelectric transducers with interdigitated electrodes as described in U.S. Pat. No. 4,697,105 to Quate et al. The existing droplet ejector technology has been used in designing various printhead configurations, ranging from relatively simple, single ejector embodiments for raster output scanners (ROS's) to more complex embodiments, such as one or two dimensional, full page width arrays of droplet ejectors for line printing. It has also found use in the synthesis of arrays of biological materials, as described in co-pending, commonly assigned applications Ser. No. 09/669,996, “ACOUSTIC EJECTION OF FLUIDS FROM A PLURALITY OF RESERVOIRS,” filed Sep. 25, 2000, Ser. No. 09/727,392, “FOCUSED ACOUSTIC ENERGY IN THE PREPARATION AND SCREENING OF COMBINATORIAL LIBRARIES,” filed Nov. 29, 2000, and Ser. No. 09/765,947, “HIGH THROUGHPUT BIOMOLECULAR CRYSTALLIZATION AND BIOMOLECULAR CRYSTAL SCREENING,” filed Jan. 19, 2001.
However, in acoustic radiation ejection, the width of the ultrasonic beam changes as a function of distance from the focusing lens. In addition, when ejecting fluids from multiple wells, it is possible that the surface of the ejected fluid could be at different heights for different wells due to manufacturing tolerances and variations in dispensed volumes. Therefore, the size of the ejected drops from different wells could show some variation, as the drop size is proportional to the width of the acoustic beam. Such a variation in drop size is generally undesirable. There is therefore, a need in the art for a method of maintaining a constant drop volume despite possible variation in the fluid height.
U.S. Pat. No. 5,268,610 to Hadimioglu, et. al. describes an acoustic drop emitting system where the drop size is modulated by varying the RF frequency by a factor of two to obtain fine gray levels in a printer system using acoustic ejection. U.S. Pat. No. 5,389,956 by Hadimioglu, et. al., demonstrates various techniques for improving drop velocity uniformity in an acoustic printing system that utilizes multiple acoustic drop ejectors. U.S. Pat. No. 5,612,723 to Shimura et al. discusses a method of altering drop size with a single fluid by variation in focal size. Shimura et al. teaches “defocusing” by changing the liquid level as a means of changing the drop volume and suggests altering the liquid level to produce a defocused beam to make larger droplets.
While above-discussed patents described some of the principles behind the invention described herein, they fail to provide a specific description of a system and method for drop size correction suitable for use in an acoustic drop ejection system utilizing multiple wells wherein there is variation in the height of the ejected fluids in different wells.
Also, the references discussed above are restricted to low F-number systems, i.e., systems in which the ratio of the distance from the focusing means to the focal point of the generated acoustic beam to the width of the acoustic beam at the focusing means is approximately about 1 or less. Unfortunately, low F-number lenses place restrictions on the reservoir and fluid level geometry and provide relatively limited depth of focus, increasing the sensitivity to the fluid level in the reservoir. For example, in bimolecular array applications the various bimolecular materials from which the array is constructed are usually contained in individual wells in a well plate. These wells often have aspect ratios of approximately 5:1, i.e., the wells are five times as deep as their width. The narrowness of the wells requires that, when F1 lenses are used, the surface of the fluid within the reservoir be no further from the lens than the width of the well. Therefore, when using an F1 lens in a 5:1 aspect ratio well, only the bottom fifth of the reservoir may be filled with fluid.
Furthermore, the prior art fails to describe any systems or methods that maintain a constant drop size when there are variations in fluid heights without the need for maintaining focus relative to the fluid surface or changing to relative position of the lens with respect to the fluid surface as the lens moves from reservoir to reservoir. Thus, there is a need in the art for a method that enables fine control of drop size in multi-well acoustic ejection applications without requiring an additional fast motion system to adjust the transducer-to-meniscus gap.